1. Field of the Invention
The invention relates generally to digital image processing and, more specifically, to a digital image processing system and method for forming halftone images using gray level image data.
2. Description Relative to the Prior Art
In the prior art, as represented by U.S. Pat. No. 5,694,224, it is known to form halftone images using gray level image data. As used herein, the term “gray level image data” refers to pixel image data whose gray level must be represented by more than one digital bit. While it is known in some systems to represent an on or off or black or white binary pixel by more than one digital bit, such representation is unnecessary but only done as a convenience for the particular hardware structure. With the gray level pixels referred to herein the gray levels, whether they be in terms of density or size, must be defined in terms of more than one digital bit and, thus, such pixels are described in varying shades of gray from either no density to maximum density and at least one shade therebetween. For example, where the gray level of a pixel is represented by a bit depth of four bits it is possible to define 15 levels of gray plus background or no color. With a bit depth of eight bits it is possible to define 255 levels of gray plus background or no color. It will be understood, of course, that colors other than gray are also contemplated as will be understood from the description below.
The above referred to prior art discloses the use of a halftoning process wherein cells or groups of gray level pixels are used to form different densities of halftone dots in accordance with various growth patterns for production of said halftone dots. In a typical color image forming process various color separation images of the color image are processed separately and subjected to a halftoning process before combining the individual color separation images in superposed registered form for example onto a receiver sheet or a display. In order to reduce the formation of artifacts such as moire artifacts in a color image, it is well known to provide some form of rotation of the various color separation screen images. Because of the nature of the rotation of the screens and differences in the colors, the processing of the respective color separation image data is more advantageously processed separately. In processing of the respective image data for the respective color separations it has heretofore been known to subject each gray level pixel to a threshold comparison to determine a halftone rendering value for rendering the current pixel in accordance with the halftoning process. In accordance with the invention, there is provided a direct lookup table method and apparatus for determining a rendered gray level halftone pixel.
The invention and its further objects and advantages will become more apparent from a discussion of the preferred embodiments of the invention.